1. Field of the Invention
The present invention relates to a toner composition for development of an electrostatic image in the electrophotographic process, electrostatic recording process, electrostatic printing process and the like.
2. Description of the Prior Art
In development of the electrostatic image in electrophotography, a toner must have high fluidity in order to form a visible image with good quality. For this purpose, fine powder of silicon dioxide (silica) has been generally used in admixture with toner powder as a surface flow auxiliary.
However, because fine powder of silica is hydrophilic when it is directly used, it absorbs moisture in the air under high temperature and high humidity conditions, which decreases the fluidity or causes aggregation of toner particles. For this reason, it has been proposed to use silica fine powder treated by a hydrophobic treatment (See the Japanese Patent Laid-Open Nos. 5782/1971 and 47345/1973). For example, a dimethyl substitution product has been known, in which a methyl group of silane is bonded with silica by a reaction of dimethyldichlorosilane with hydrophilic silica (R-972: Nippon Aerosil Co., Ltd.).
However, the fine powder of silica is not hydrophobic enough even if it has been treated to have a hydrophobic property. Aggregation property is noted at high temperature and high humidity and the fluidity of the toner is decreased. Thus, the degree of hydrophobic property has become an important issue.
Specifically, in case of R-972, for example, the silanol group of hydrophilic silica is turned to hydrophobic only by 70 to 80% while the remaining 20 to 30% of silanol groups are not substituted and remain unchanged. The degree of hydrophobic property of R-972 is thus only 40.
Therefore, it has been pointed out that, when silica fine powder with such a degree of hydrophic property is used, it is difficult to stably form a great number of visible images with good quality for a long period by the toner.
More recently, there have been several proposals to solve these problems. For instance, the stable formation of a visible image with good quality in forming a great number of visible images for a long period can be obtained when hydrophobic silica fine powder having a hydrophobic index (i.e. a degree of hydrophobic property) of 50 or more, or more preferably 65 or more, which is obtained through a hydrophobic treatment of organic silicon compounds having a specific organic group, is added and mixed with toner powder in an amount of 0.01 to 15% by weight (Japanese Patent Laid-Open No. 81650/1984). There is also the proposal to contain 0.01 to 20% by weight of hydrophobic silica fine powder obtained through a hydrophobic treatment so that the degree of hydrophobic property is within the range of 30 to 80 (Japanese Patent Laid-Open No. 231552/1984).
Such a hydrophobic treatment has been used in the method already known, in which a chemical treatment is performed by an organic silicon compound reacting or physically adsorbing with silica fine powder. In general, a method is adopted, by which a treatment is performed by an organic silicon compound during or after the time the silica fine powder obtained by a vapor phase oxidation of a silicon halogen compound has been treated by a silane coupling agent.
However, hydrophobic silica with high hydrophobic property known in the past has a hydrophobic degree of below 80 at most. Indeed those hydrophobic silicas described in the above patent publication (Japanese Patent Laid-Open No. 231552/1984) have only a hydrophobic degree of up to 74.
The Japanese Patent Laid-Open No. 81650/1984 describes the compound with a degree of hydrophobic property of more than 65 as a high hydrophobic compound, whereas the upper limit is not clear, and it is also not known how high the hydrophobic property of the compound disclosed in the above patent publication is obtained. The hydrophobic silica having the hydrophobic degree of below 80 at most shows the improvements in electric charge retainability and fluidity compared with the conventional dimethyl substitution product having the hydrophobic degree of 40 to 42. This was not sufficient for the purpose, however, under high temperature and high humidity conditions because electric charge retainability and fluidity decreased or the stable formation of a visible image with good quality was hindered.
In case that the degree of hydrophobic property is not enough, a number of unreacted silanol groups remain in the hydrophobic silica, or in case that the substitutents reacted with silanol groups are small groups of atoms as a whole, a stable hydrogen bond is formed by carboxyl group in the binder resin of toner particles and moisture in the surroundings with the other unreacted silanol groups. As the result, the above problems arise under high temperature and high humidity conditions.
Therefore, whether the degree of hydrophobic property is enough or not is determined by which kind of hydrophilic groups the binder resin has.
As the binder resin for a toner, various types of resins are used including styrene type copolymers such as polystyrene, styrene-butadiene copolymer, styrene-acrylic copolymer, etc., ethylene type copolymers such as polyethylene, ethylene-vinyl acetate copolymer, etc., poly-(meth)acrylic acid ester, polyester resin, epoxy resin, and polyamide resin, etc. Of these resins, those having a naturally high hydrophobic property such as normal styrene-acrylic resin, a high degree of hydrophobic property is normally not required of the silica. Above all, however, a polyester resin is obtained by condensation polymerization of alcohol and carboxylic acid. Because many carboxyl groups, which are hydrophilic groups, are contained in this resin, hydrogen bonds of such groups with water cause the decrease of electric charge retainability and fluidity of the toner. Thus, it has been pointed out that the degree of hydrophobic property is not sufficient.
When a polyester resin is used as a major component of a binder resin, a polyester resin having an OHV/AV of 1.2 or more is commonly used wherein AV is an acid value of a polyester resin and OHV is a hydroxyl value thereof. The reason for this is as follows: the lowest fixing temperature of the toner obtained from a polyester resin having the OHV/AV value of below 1.2 is higher than that of the toner obtained from a polyester resin having the value of 1.2 or more. Further, it has lower fluidity, and it is necessary to add a large quantity of a surface flow auxiliaries such as hydrophobic silica fine powder to obtain sufficient fluidity.
Also, in the coventional type hydrophobic silica, it is necessary to add more quantity of hydrophobic silica to maintain the fluidity of toner particles. For example, in the above patent publication (Japanese Patent Laid-Open No. 81650/1984), which describes the compound with a hydrophobic index of 50 or more, it is proposed to add hydrophobic Silica in an amount of 0.01 to 15 % by weight. In the above patent publication (Japanese Patent Laid-Open No. 231552/1984) describing the compound with a hydrophobic index of 30 to 80, it is proposed to add hydrophobic silica in an amount of 0.01 to 20% by weight.
However, there remained the problems that, if the addition amount of silica is increased, the isolated silica gives damage to the surface of a photoconductor drum and the silica causes black spots as the initiator. The black spot is a type of filming on a photoconductor drum and it appears as black points on a visible image. Because the particles of hydrophobic silica are considerably hard, this phenomenon is remarkably appeared when the photoconductor drum used is a substance of relatively low hardness such as a selenium-tellurium type or an organic photoconductor drum. Further, the same problem occurs even in case of a selenium-arsenic type substance, which is relatively hard but is brittle to mechanical shock.
Accordingly, it is preferred that the addition amount of silica be as low as possible, and it is also preferred to use such hydrophobic silica, which can improve electric charge retainability and fluidity of the toner by adding it in such very small quantity.
On the other hand, a hydrophobic treatment of silica has been performed in the past through the volatile silanes in a reactor heated at about 400.degree. C. For example, a method to utilize the thermal decomposition oxidizing reaction in oxyhydrogen flame of silicon tetrachloride gas has been used, wherein the following reaction occurs: EQU SiCl.sub.4 +2H.sub.2 +O.sub.2 .fwdarw.SiO.sub.2 +4HCl
In the meantime, because it is not very easy to remove hydrogenchloride generated during the reaction, it has been pointed out that the pH value of the hydrophobic silica thus obtained decreases to about 3 to 2, and the problems such as the rusting on inner wall of the tank for hydrophobic silica in the toner facilities during long-term use arise.
Specifically, the conventional hydrophobic silica obtained in the past had the various problems such as the suitability of the degree of hydrophobic property and the amount to be added, and in addition to these problems the counter measures are urgently needed to improve the acidification of hydrophobic silica fine powder caused by a hydrogen chloride generated during the treatment.